The invention relates to a method for X-ray exposure control, notably for exposures carried out during dynamic X-ray examinations of an object, that is, exposures during which the X-ray absorption of the object changes and/or the object moves. The invention also relates to an X-ray generator provided with an automatic exposure control device for carrying out such a method.
For X-ray examination of the human body and its organs it is necessary to carry out a large number of settings for an X-ray generator so as to achieve an optimum exposure of the examination zone. This is due to the fact that the density of the various organs or regions of the body is very different per se and also differs from one person to another, that is, in dependence on the size and the weight of the relevant person. In order to ensure a safe examination of the patient while applying an as small as possible radiation dose, furthermore, in practically all countries there are official regulations which allow given parameters to be adjusted or changed only within given limits.
Attention should be paid in particular to the following parameters which are dependent on one another, have to be adapted to one another and influence the acquired image each time in a different way.
On the one hand, the dose rate of the X-ray tube (that is, essentially the exposure kV voltage) determines the contrast and the contrast range of the objects imaged. The radiation dose, however, first of all determines the signal-to-noise ratio of the image whereas, in order to optimize the image sharpness notably in the case of moving objects, the exposure time may not exceed a given maximum value. Furthermore, for the adaptation or selection of these parameters it is also necessary to take into account the density (X-ray absorption) of the object to be examined, that is, generally speaking, the thickness of the patient. Finally, various legal rules and regulations apply also to the X-ray dose that is incident on a radiation receiver.
Various methods and devices are known for the partial automation of the adjustment of these parameters. For example, EP 0 073 644 describes a method in which, after preselection of given secondary conditions such as relative hardness number, exposure field format, film and foil sensitivity etc., first the X-ray exposure is carried out with a programmed tube voltage and a programmed tube current until a programmed dose is reached, and the time elapsing until that instant is measured. The X-ray exposure is then continued while using for the X-ray voltage and the mAs product a value which is associated with said measured time and is stored. The dose and/or the dose power of the X-rays is thus adapted to the density of the object to be imaged (object transparency).
This and other partly automated methods and devices whereby one of said parameters is determined in dependence on the other, preset or measured parameters, have the drawback that the image quality is dependent to a significant degree on the skills of the operator in suitably finding the relevant presetting. These methods and devices often reach their limits also in the case of dynamic processes where, for example exposures are to be performed with a moving contrast medium, because notably in cases where the dose rate is not optimally adjusted, a prolongation of the exposure time which is automatically imposed by a change of absorption may lead to lack of sharpness in the image.
Therefore, it is an object of the invention to provide a method for X-ray exposure control whereby the image quality can be further enhanced, that is, notably in the case of dynamic examinations of the kind set forth.
Furthermore, it is also an object of the invention to provide an X-ray generator which includes an automatic exposure control device for carrying out such a method and in which the degree of automation is further increased and hence the image quality is no longer dependent on the skills of an operator to such a high degree.
This object is achieved in conformity with claim 1 which discloses a method of the kind set forth which includes the following steps: presetting for an exposure a maximum exposure time (Tmax) which may in principle may not be exceeded; presetting an exposure kV start voltage for an X-ray tube in dependence on an object to be examined; starting the X-ray exposure and measuring an X-ray absorption of the object; controlling the exposure by changing the exposure kV start voltage at the maximum exposure time (Tmax) when the X-ray absorption is higher than or equal to a first threshold value (B), or controlling the exposure by changing the exposure time at a constant exposure kV start voltage when the X-ray absorption is less than the first threshold value (B).
xe2x80x9cPresettingxe2x80x9d a maximum exposure time Tmax as well as an exposure kV start voltage is in this context is to be understood to mean presetting by an operator as well as presetting carried out, for example, by a microprocessor unit in dependence on other input data, or a fixed programming of the two variables. This also holds for all further possibilities for adjustment explained hereinafter.
In conformity with claim 4 this object is also achieved by means of an X-ray generator which is provided with an automatic exposure control unit for carrying out such a method, wherein the automatic exposure control unit includes a multiple controller for controlling an X-ray tube, which multiple controller includes with a dose controller and at least one dose rate controller which are subject to a dose rate sensor for measuring an X-ray absorption of the object.
A special advantage of these solutions resides in the fact that, notably in the case of dynamic examinations during which the absorption of the object to be examined changes, the risk of unsharp images due to an excessive exposure time is avoided. Moreover, when a suitably fast switching technology is used, the control of the exposure kV start voltage (that is, generally speaking, the organ kV voltage of an organ to be examined) can be terminated already after from approximately 1 to 2 ms, so that the exposure is then carried out with an essentially constant exposure kV voltage when the absorption remains constant during the exposure.
The dependent claims relate to advantageous further versions and embodiments of the invention.
The versions in conformity with the claims 2 and 3 enable the method to be adapted even better to given examination conditions or object properties by way of further ranges of operation.
The embodiment disclosed in claim 5 enables very fast control of the dose rate in both directions.
The embodiments disclosed in the claims 6 and 7 enable adjustment of a maximum and a minimum exposure time, respectively, for an exposure. The embodiment disclosed in claim 8 enables adjustment of a reference value for the dose or the dose power, whereas the embodiment disclosed in claim 9 enables adjustment of a start value for an exposure kV voltage whose control range as well as maximum value are adjustable.